With the ever increasing use of ultrasound in clinical work and the development of progressively more sophisticated systems of tissue visualization, the importance and the urgency of specifying quantitatively the maximum safe dosage level, the possible hazards or toxic effects, and the mechanism of their production is obvious. It is essential that this information be available for each of the modes of application of ultrasonic energy to each of the different organs and systems of the human organism under different physiological and pathological conditions - an obviously monumental task, since the damage thresholds may vary from one tissue to another, from species to species and with the physiological state of the animal or with the disease process. It is proposed to obtain quantitative and statistically significant data from intensive studies of the threshold ultrasonic "dosage" parameters resulting in irreversible structural (histological and ultrastructural) damage to the central nervous system of adult cats since it has been used by most of the workers in this field and thus already has more data available than any other system. Moreover, since in the adult animals the neurons, being fixed postmitotic cells, do not replicate, the central nervous system may provide a more definitive indication of irreversible morphological damage than any other tissues with differentiating intermitotic or reverting postmitotic parenchymal cells. This data will be used to refine an analytic model which has been found to hold considerable promise in predicting occurence of damage in terms of measurable physical parameters of tissues and of ultrasonic fields. Instrumentation for the measurement of the acoustic power in ultrasonic pulses will be developed. Both of the diagnositic modes of ultrasonic irradiation -- pulsed and C.W. (as used in Doppler and holographic techniques) -- will be studied. Extrapolability of the model will be evaluated in organs with different physical and biological properties (fatigue strength, thermal diffusivity and heat transfer coefficient due to blood flow, anisotropy, thermal injury thresholds) and with different types of parenchymal cells (viz. liver, testes) as their ultrasonic sensitivity may differ in the same manner as their morphological radiosensitivity.